275/60/15 woes

I've read you can fit 275/60/15s on a 72 Demon if you nail the back spacing. Looks like I missed it. 15x8s with 4.5 back space. That seemed to be the magic formula. But my car is small bolt pattern with the standard 10 inch drums. Maybe that's the difference. Thoughts? Options? Really limited on wheel options for small bolt pattern. (Btw the back of the car is sitting a little higher because of the weight distribution on Jack stands.) Will probably throw some 255s on the old wheels until I can upgrade to big bolt pattern.

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